1. Field of the Invention
The present invention relates to a signal processing apparatus and a signal process method for performing at least a frequency analysis on a response signal obtained as a result of outputting a measurement signal in a system for measurement. The present invention further relates to a computer program executed in such signal processing apparatus, and a signal generation method of generating a measurement signal.
2. Description of the Related Art
In audio systems of related art for reproducing and outputting an audio signal, a measurement signal such as a time stretched pulse (TSP) is emitted from a loudspeaker and then picked up by a microphone. Based on such pickup sounds, frequency-amplitude characteristics and travel time between loudspeaker and microphone in a system are measured.
The TSP signal is generated to satisfy at least the following conditions. Let “N” represent the number of samples of a signal and “Fs” represent a sampling frequency (operation clock frequency), and a signal ranging from 0 Hz to Fs/2 Hz is contained in steps of Fs/N Hz at the same gain level.
For example, given the sampling frequency Fs=48 kHz and the number of samples N=4096, a signal ranging from 0 Hz to 24 (48/2) kHz is contained in steps of about 11.7 (48000/4096) Hz on the frequency domain at the same gain level.
If a signal satisfying only this condition is output in a wave in the time domain as a measurement signal, that signal has a very short duration of time and a low energy level. In the measurement signal generally called TSP signal, a predetermined frequency component of the measurement signal is phase-rotated depending on frequency. With the phase rotation performed, and the signal as a time-domain wave has energy spread in the time domain.
On the other hand, a phase-rotated signal tends to become small in the amplitude thereof. The phase-rotated signal is thus increased in gain (volume) to a level required for measurement.
OA-TSP (optimized Aoshima's TSP) signal is well known as one example of TSP (as described in Japanese Unexamined Patent Application Publication No. 3-6467). A signal satisfying the following equations (1) and (2) in the frequency domain is inverse Fourier transformed to be time-domain waveform.H(n)=a0·exp(j4mπn2/N2), 0≦n≦N/2  (1)                (m and n: integers)H(n)=H*(N−n), N/2+1≦n≦N−1  (2)        (n: integer and *: conjugate)        
FIG. 20 illustrates an OA-TSP signal with the number of samples N=4069 and m=2048. As shown in FIG. 20, the amplitude of the signal is normalized to 1.0.
The TSP signal of FIG. 20 is emitted from a loudspeaker and the emitted sound is then picked up by a microphone. Based on the collected sound, acoustic characteristics such as frequency-amplitude characteristics and travel time between the loudspeaker and the microphone are measured.
In order to increase a signal-to-noise (S/N) ratio in such an acoustic measurement, the TSP signal is periodically reproduced and the response waveform of the TSP signal is synchronization addition/averaged by a unit of period (equal to 4096 samples) in a general practice.
Frequency-amplitude characteristics are obtained by frequency analyzing the measured TSP response signal using fast Fourier transform (FFT). The frequency-amplitude characteristics include a combination of transfer functions Hsp, Haco and Hmic of a loudspeaker, measurement space and a microphone.
The linear or periodical convolution of the response signal and an inverse filter (inverse TSP signal) defined by the following equations (3) and (4) (representing conditions in the frequency domain) results in accurate phase information of a transfer function. The impulse response is determined by performing inverse fast Fourier transform (IFFT) on the signal and the inverse filter to return the signal to a time domain signal.H−1(n)=(1/a0)·exp(−j4mπn2/N2), 0≦n≦N/2  (3)H−1(n)=H*(N−n), N/2+1≦n≦N−1  (4)
One example of resulting impulse response is shown in FIG. 21 for reference purposes only.
By analyzing the impulse response, the travel time between the loudspeaker and the microphone is measured.
In the audio system, the acoustic measurement result thus obtained is accurately used in sound field correction function.
More specifically, the frequency-amplitude characteristics (also simply referred to as frequency characteristics) are used as an evaluation indicator for use in adjusting an equalizer so that current characteristics becomes flat in the frequency domain (or becomes any frequency curve).
Gain information in an environment can be calculated from the frequency-amplitude characteristics. The term gain contains information relating to the efficiency of the loudspeaker and sound absorption and reflection characteristics of walls, and is typically calculated from an average level of a particular band for an intended purpose of the frequency characteristics.
A recommendation for the use of a bass management system is presented or the bass management system is automatically set. In the bass management system, low-frequency reproduction performance of the loudspeaker in use is analyzed and determined from the frequency characteristics and a low-frequency signal of a source content is sent to a sub-woofer.
Information regarding the distance between the loudspeaker and the microphone is obtained from information regarding sound travel time between the loudspeaker and the microphone acquired from the impulse response. Delay time adjustment (time alignment) can be performed on the sound emitted from the loudspeaker based on the distance information.
Variations in the performances of the loudspeakers installed in room space, variations in the distance to the position of a listener (microphone position) and variations in the environment (such as closeness to walls and the presence of obstacles) are corrected in the sound field correction process based on the acoustic measurement. In this way, the process allows the user to listen to a correct sound image as a creator of each content intends.
The audio system automatically performs the sound correction process in response to a user operation input. Such an automatic sound correction function is an extremely effective function because it is complicated and difficult for the user to set and modify manually a variety of parameters, particularly in a multi-channel system having a plurality of loudspeakers and it is difficult to prepare a plurality of loudspeakers having the same characteristics.
The sound correction requires that the measurement signal (response signal) be frequency-analyzed to acquire the frequency-amplitude characteristics. The problem of frequency resolution in the frequency analysis during the acoustic measurement has been pointed out.
FIG. 22 illustrates frequency analysis results obtained from the TSP signal having the number of samples N=4096 and the sampling frequency Fs=48 kHz. As shown in FIG. 22, the abscissa represents frequency (Hz) and the ordinate represents gain (dB).
As previously discussed, given the number of samples N=4096 and the sampling frequency Fs=48 kHz, the frequency resolution in the frequency analysis results is 11.7 Hz from Fs/N=48000/4096.
The frequency resolution is 11.7 Hz over the entire range. In accordance with the human auditory sense, the frequency axis is logarithmically represented as shown in FIG. 22. In medium to high frequency regions labeled the letter “B,” the frequency resolution becomes higher. On the other hand, the low frequency region labeled the letter “B,” the frequency resolution becomes lower.
Several multi-channel systems having a sub-woofer use the bass management system in the low frequency region. The lower the frequency, the lower the frequency resolution becomes. It may be difficult to determine appropriately whether to send a signal to the sub-woofer. The sound correction cannot be performed in a proper manner.
The frequency resolution is represented by Fs/N as previously discussed. An increase in the value N, namely, in the number of samples in the time domain of the TSP signal increases the frequency resolution. For example, if the number of samples N is doubled as 4096×2=8192, the frequency resolution becomes 5.85 Hz from 48000/8192.